NEW: Pakistan wants to push Taliban south so it can encircle militants, expert says

U.N.: World must launch "massive effort" to care for Pakistani refugees

Some 1.5 million people have been displaced by 10 months of fighting

ISLAMABAD, Pakistan (CNN) -- The Taliban are digging in for a "bloody urban battle" against the Pakistani army in a hotly disputed city in the western part of the country, a strategic expert warned Thursday.

A Pakistani army spokesman says the military offensive has caused many to leave their homes for camps.